Individual interleaving of data streams for mimo transmission

ABSTRACT

The present invention, generally speaking, provides interleavers and methods of interleaving that satisfy the need for backward compatibility while effectively addressing competing design objectives. In accordance with one aspect of the invention, data is transmitted using a number of transmit antennas greater than an expected number of receive antennas. At least one pair of transmit antennas is formed, and multiple second data streams are formed from a first data stream, successive bits in said first data stream being assigned to different ones of said second data streams. Block interleaving of multiple respective ones of said second data streams is individually performed. During successive transmission intervals, the pair of transmit antennas is used to transmit a pair of data symbols taken from different ones of said second data streams, followed by an equivalent transformed pair of data symbols.

The present invention relates to wireless digital communications.

A block diagram of a typical 802.11a/g transmitter is shown in FIG. 1.Such a transmitter is a Single-Input-Single-Output (SISO) system. Bitsto be transmitted are applied to a forward error correction (FEC)encoder 101, followed by a interleaver 103. Output bits of theinterleaver 103 are grouped and mapped within the signal plane by asymbol mapper 105 (e.g., a QAM mapper) to form symbols. An IFFToperation 107 then follows in which symbols are mapped to a series ofsubcarrier frequencies (i.e., frequency bins) and transformed to obtaina series of time samples. A cyclic extension operation 107 (equivalentto adding guard symbols) is performed to obtain a resulting OFDM symbol.Pulse shaping 109 and IQ modulation 111 are then performed to obtain anRF output signal 113.

A typical 802.11a/g system has a block interleaver (e.g., blockinterleaver 103) that may be described in terms of a first permutationfollowed by a second permutation using the following parameters:

N_CBPS is the size of the interleaver, i.e., the number of coded bitsper symbol

k is the index of the input bits

i is the index after the first permutation

j is the index after the second permutation

The first and second permutations are as follows:

1^(st) permutation

i=(N_CBPS/16)(k mod 16)+floor(k/16), k=0,1, . . . , N_CBPS-1

there are 16 columns and N_CBPS/16 rows

bits are written row by row and are read column by column

2^(nd) permutation

j=s*floor(i/s)+(i+N_CBPS−floor(16*i/N_CBPS)) mod s, i=0,1, . . . ,N_CBPS-1

where s=max(N_BPSC/2,1), N_CBPS is the number of bits per symbol in theOFDM subcarrier. For different columns, the bit significance index ischanged so that the adjacent bits are not always mapped to the sameindex in any symbol.

The foregoing permutations are represented by blocks 201 and 203 in FIG.2.

Following the strong market success of 802.11a/b/g wireless networking,an 802.11n working group was formed in 2003, chartered to create astandard for high-throughput wireless LAN. In this proposed standard,the maximum data rate can go as high as

720 Mbps with more than twice the range as compared to 802.11a/b/g. Thefundamental technology is called Multiple-Input-Multiple-Output (MIMO),which essentially uses multiple antennas to exploit path diversity inthe wireless medium. When discussing a MIMO system, M×N means M transmitantennas and N receive antennas.

Multiple antennas makes possible a type of coding referred to as SpaceTime Block Coding (STBC), an example of which is Alamouti coding. InSTBC, a block of information is encoded and transmitted over multipleantennas (space) and over multiple symbol periods (time).

It is desirable that 802.11n (MIMO) systems be backwardly compatiblewith at least 802.11a/g (SISO) systems. With respect to interleaving inparticular, a need exists for interleaving arrangements that achievebackward compatibility while addressing competing design objectives(e.g., compactness, low power consumption, and robustness ofcommunications).

“Space-time frequency coding for OFDM based WLANs” to Oteri ET ALdiscloses a transmitter using the 802-11 a/g standard comprising a bitencoder for encoding an input data stream, a puncturizer forpuncturizing the encoded data, and a multiplexer for separating theencoded and puncturized input data stream into two data streams based onthe parity of the bits of the coded data stream output from the encoder,an interleaver for processing each data stream, means for cyclicallyshifting one of the two streams, and two antennas for transmitting oddbits of the coded data and even bits of the coded data separately.

EP 1 351 414 discloses a physical layer configuration for a wirelessnetwork, each of the stations of the network implementing one ofdifferent Physical configurations, including antenna configurationsamongst SISO, 2×2 MIMO, or 4×4 MIMO, for informing in advance receiverswhat configuration should be used for receiving or transmitting data.

US 2002/085643 discloses a MIMO system comprising a plurality oftransmit antennas that provide polarization diversity and which arepositioned such that spatial diversity is avoided, the system mayprovide non-MIMO communication simultaneously with MIMO communications.

The present invention, generally speaking, provides interleavers andmethods of interleaving that satisfy the need for backward compatibilitywhile effectively addressing competing design objectives. In accordancewith one aspect of the invention, data is transmitted using a number oftransmit antennas greater than an expected number of receive antennas.At least one pair of transmit antennas is formed, and multiple seconddata streams are formed from a first data stream, successive bits insaid first data stream being assigned to different ones of said seconddata streams. Block interleaving of multiple respective ones of saidsecond data streams is individually performed. During successivetransmission intervals, the pair of transmit antennas is used totransmit a pair of data symbols taken from different ones of said seconddata streams, followed by an equivalent transformed pair of datasymbols. In accordance with another aspect of the invention, a method isdefined in claim 1. [OPERATION C]. In accordance with another aspect ofthe invention data is transmitted using a number of transmit antennasgreater than an expected number of receive antennas. A group of transmitantennas is formed, and multiple second data streams are formed from afirst data stream, including a second data stream for each of theantennas, successive bits in said first data stream being assigned todifferent ones of the second data streams. Block interleaving ofmultiple respective ones of said second data streams is individuallyperformed. During successive transmission intervals, respective nonzerosymbols are output in turn for transmission from different ones of saidantennas such that during a given transmission interval a non-zerosymbol is assigned to just one antenna of the group of antennas and zerosymbols are assigned to other antennas of the group of antennas.

The present invention may be further understood from the followingdescription in conjunction with the appended drawing. In the drawing:

FIG. 1 is a block diagram of a known SISO communication transmitter.

FIG. 2 is a more detailed block diagram of the interleaver of FIG. 1.

FIG. 3 is a block diagram of a portion of a MIMO communicationtransmitter.

FIG. 4 is a block diagram of a portion of a communication transmitterusing tone-interleaved signals for two antennas.

FIG. 5 is a block diagram of a portion of a communication transmitterusing tone-interleaved signals for two antennas in accordance with oneaspect of the present invention.

FIG. 6 is a block diagram of a portion of a communication transmitterusing tone-interleaved signals for two antennas in accordance withanother aspect of the present invention.

FIG. 7 is a block diagram of a portion of a communication transmitterusing Alamouti coding.

FIG. 8 is a block diagram of a portion of communication transmitterusing OFDM and Alamouti coding.

FIG. 9 is a block diagram of a portion of a communication transmitterusing Alamouti coding in accordance with one aspect of the invention.

FIG. 10 is a block diagram of a portion of a communication transmitterusing OFDM and Alamouti coding in accordance with one aspect of theinvention.

In the following description, the case of two transmit antennas is shownas being exemplary of the more general case of N transmit antennas. Theprinciples of the present invention may readily be extended from twoantennas to more than two antennas as will be appreciated by those ofordinary skill in the art.

For 802.11n, multiple spatial streams are required. Since for an 802.11nsystem to be backward compatible with an 802.11a/g system, the 802.11a/ginterleaver has to be present. The present approach is to create newinterleavers based on the 802.11a/g interleaver. That is, the input bitsare parsed to two streams, and on each stream an 802.11a/g interleaveris used.

Referring now to FIG. 3, a block diagram is shown of a MIMOcommunication transmitter. A single information stream is applied to abit parser 301. Depending on the transmission mode, the bit parserproduces a single information stream or two separate informationstreams. In SISO mode, the bit parser steers the incoming informationstream to an upper branch 311 of an interleaver 310. The upper branch ofthe interleaver may have the same construction as the interleaver ofFIG. 2. That is, a block interleaver operation 313 is followed by asignificance index shuffler 315. In MIMO mode, the bit parser outputsalternate bits of the incoming information stream to alternate ones ofthe upper branch 311 of the interleaver and a lower branch 312 of theinterleaver, producing two separate information streams.

The lower branch of the interleaver preferably includes correspondingblocks 314 and 316 as the upper branch of the interleaver. In addition,the lower branch of the interleaver includes a block 316 c (Operation C)and may optionally include a block 316 b (Operation B) or a block 316 a(Operation A).

It is desirable to separate adjacent bits, now in different spatialstreams, as apart as possible in the frequency domain. One simple way ofdoing it is to cyclically rotate the output of block 316 in themultiples of N_CBPS (Operation C). Operation C may be imagined in termsof buffering the interleaved block in a linear buffer and performingcyclic rotation by a multiple of N_CBPS. Using a realistic system model,it may be shown that for a 2×2, 40 MHz system, cyclically rotating57*N_CBPS, i.e. cyclically rotating 57 frequency tones in OFDM, wouldgenerate the lowest PER (Packet Error Rate) for a given SNR. For a 2×2,20 MHz system, a suitable rotation is 25*N_CBPS. Note that in OperationC the bit significance index has not been changed.

It is desirable also to change the bit significance index as between thetwo streams. There are many ways to do so. One way is to changeOperation 2 (block 316), for example by changing the definition of s inthe second permutation above. Alternatively, since the currentpermutation changes according to the column index, changing the bitsignificance index may be achieved by performing the permutationaccording to a different column index, say column index+1.

To avoid modifying Operation 2 (in view of hardware reuseconsiderations, for example), an equivalent effect can be achieved atvarious locations within the circuit, e.g., at Bit Parser 301 orOperation A or Operation B. One simple way of implementing Operation B,for example, is to cyclically rotate bits belonging to a symbol of thesecond bit stream, say by 1. In the case of a third bit stream, bitsbelonging to a symbol would be cyclically rotated by 2, etc.

Operation A may take the form of another interleaver, for example,designed so as to achieve distinct significance index shuffling. It isalso potentially possible to combine Operation A with the Bit Parsingblock 301.

Distinct significance index shuffling can also be done as part ofOperation C (i.e., on top of what has already been done to achievefrequency separation). A simple way to do so is to shift one more bit inthe second bit stream, two more bits in the third bit stream, etc.

When transmit antennas outnumber receive antennas in a MIMO-OFDM system,the number of data streams must be fewer than the number of transmitantennas. However, it is known that the additional transmit antenna(s)can provide added spatial diversity and thus further improve the systemperformance. One way of doing so is to use spatial spreading, which usesthe signal cyclic-delayed from the other antenna's signal.

Another other way of doing so is to use tone-interleaved signals for twoantennas as shown in FIG. 4. In FIG. 4, blocks 401, 403 and 405correspond generally to blocks 101, 103 and 105. Block 407 performs toneinterleaving in the following manner:

In the frequency domain,

ant1=[a1, 0, a3, 0, . . . ]

ant2=[0, a2, 0, a4, . . . ]

That is, a pair of antennas is formed and during a particular symbolperiod, half of the tones within an OFDM symbol transmitted via oneantenna of the antenna pair are used and half of the tones are unused.In the case of the other antenna, the use or non-use of a particulartone is reversed. This simple tone interleaving does not fully exploitthe frequency diversities in the OFDM signal as the result of the simplealternating scheme.

Referring to FIG. 5, it is assumed that there are N antennas but onlyone stream is allowed. A single information stream is applied to an FECencoder 501 followed by a bit parser 503. The bit parser outputs bits ofthe incoming information stream in turn to different ones of thebranches 510 a, . . . , 510 n. Each branch includes an interleaver 511followed by a bit-to-symbol mapper 513 and a tone interleaving block515. Frequency diversity is exploited as follows:

ant_(—)1=[a1, 0, . . . 0, a_N+1, 0, . . . ]

ant_(—)2=[0,a2,0, . . . 0,a_N+2,0, . . . ]

ant_N=[0, . . . , 0,a_N−1,0,a_N+N,0, . . . ]

Interleaver depth can be adjusted to meet latency requirements.

Referring to FIG. 6, the same structure can be applied to STBC, withSTBC (block 617) being applied after tone interleaving.

For a 2×1 system, a particular variant of STBC is Alamouti coding.Alamouti coding maps two adjacent symbols to two transmit antennas forsimultaneous transmission. To take full advantages of Alamouti Coding(AC), an interleaver is usually utilized before AC. Referring to FIG. 7,data to be transmitted is applied to an FEC encoder 701, followed inturn by an interleaver 703, a QAM mapper 705 and a symbol parser 707.The symbol parser produces multiple symbol streams, which are applied toa AC block 709.

In an OFDM system, as illustrated in FIG. 8, for each branch an IFFT 806is added following the AC block 809.

For 4×1 system, Alamouti Coding is generalized to 4×1 Space-Time BlockCode (STBC). The current common scheme works as follows:

s1(k)-s2*k) repeat

s2(k) s1*(k) repeat

s3(k)-s4*(k) repeat

s4(k) s3*(k) repeat

Each line represents symbols transmitted on a particular antenna duringtwo successive symbol periods. More particularly, during a first of twosuccessive symbol periods, distinct symbols are transmitted on antennas1 through 4. During the next successive symbol period, equivalent buttransformed symbols are transmitted. Hence, the negative conjugate ofthe symbol that was transmitted on antenna 2 is transmitted on antenna1, the conjugate of the symbol that was transmitted on antenna 1 istransmitted on antenna 2, etc. The indicated pattern is repeated (i.e.,for antenna 1, there follows s1(k+1),

Coded bits in close proximity should be separated as far apart aspossible in the time domain and in the frequency domain when OFDM isapplied. Although it is possible to design an interleaver that wouldachieve this goal, it is impossible to upgrade a system with an existinginterleaver. Also, for M×1 systems where M>2, the existing repetitivescheme does not fully utilize different varieties of spatial diversity.

To better utilize different varieties of spatial diversity, multipleinformation streams are formed, which are then individually interleaved.Referring to FIG. 9, blocks 901, 903, 911 a, 911 b, 913 a and 913 bcorrespond generally to blocks 501, 503, 511 a, 511 b, 513 a and 513 b.An AC block 915 receives the resulting streams (individuallyinterleaved) and performs Alamouti Coding thereon in a known manner.This arrangement may be referred to as Individually Interleaved AlamoutiCoding (I2AC).

In an OFDM system, as illustrated in FIG. 10, for each branch, an IFFT1006 is added following the AC block.

For M>2, spatial rotation may be applied on top of I2AC. For the 4×2case, for example, four streams will be bit parsed. Each stream isindividually interleaved and mapped to QAM symbols. AC coding is thenperformed as follows:

s1(k)-s2*(k) s1(k+1) -s3*(k+1) s1(k+2) -s4*(k+2)

s2(k) s1*(k) s2(k+1) -s4*(k+1) s2(k+2) -s3*(k+2)

s3(k)-s4*(k) s3(k+1) s1*(k+1) s3(k+2) s2*(k+2)

s4(k) s3*(k) s4(k+1) s2*(k+1) s4(k+2) s1*(k+2)

So the streams are pair-wise STBC-ed (three pairs possible ((1,2)(3,4)), ((1,3),(2,4)), ((1,4),(2,3))) over the 4 antennae. This can bedone in two ways as follows:

Option 1:

The Alamouti encoding is done over six consecutive OFDM symbols asfollows: the first two OFDM symbols use the combination (1,2),(3,4) onall the frequencies, the next two OFDM symbols use the combination(1,3),(2,4) over all frequencies and the last two OFDM symbols use(1,4),(2,3) over all frequencies, and then the pattern repeats for thenext six OFDM symbols. The disadvantage of doing it this way is that thechannel matrix for each frequency changes with time.

Let, aij, bij, cij and dij be the jth data-symbol in the ith OFDM blockand the four streams are denoted by a,b,c and d. Let each OFDM blockhave N data symbols. A set of symbols between square brackets [ ] is oneOFDM symbol. The operations performed by the AC block may then berepresented as follows:

Input: to STBC (AC) block: [a11 a12 . . . a1N] [a21 a22 . . . a2N] [a31a32 . . . a3N] . . . [b11 b12 . . . b1N] [b21 b22. . . b2N] [b31 b32 . .. b3N] . . . [c11 c12 . . . c1N] [c21 c22 . . . c2N] [c31 c32 . . . c3N]. . . [d11 d12 . . . d1N] [d21 d22 . . . d2N] [d31 d32 . . . d3N] . . .Output of STBC block: [a11 a12 . . . a1N] [−b11* −b12 . . . −b1N*] [a21a22 . . . a2N] [−c21* −c22 . . . −c2N*] [a31 a32 . . . a3N] [−d31* −d32. . . d3N*] . . . [b11 b12 . . . b1N] [a11* a12* . . . a1N*] [b21 b22 .. . b2N] [−d21* −d22 . . . −d2N*] [b31 b32 . . . b3N] [−c31* −c32 . . .−c3N*] . . . [c11 c12 . . . c1N] [−d11* −d12* . . . −d1N*] [c21 c22 . .. c2N] [a21* a22* . . . a2N*] [c31 c32 . . . c3N] [b31* b32* . . . b3N*]. . . [d11 d12 . . . d1N] [c11* c12* . . . c1N*] [d21 d22 d2N] [b21*b22* . . . b2N*] [d31 d32 . . . d3N] [a31* a32* . . . a3N*] . . .

Option 2:

The Alamouti encoding is done over two OFDM symbols as follows: thefirst frequency bin uses combination (1,2),(3,4), the 2nd frequency binuses combination (1,3),(2,4), the 3rd frequency bin uses (1,4),(2,3) andthe pattern repeats. Hence the Alamouti encoding uses symbols fromdifferent antennae for each frequency bin. However the channel matrixfor each frequency bin does not change over time.

Input to STBC block: [a11 a12 a13 a14 . . . a1N] [b11 b12 b13 b14 . . .b1N] [c11 c12 c13 c14 . . . c1N] [d11 d12 d13 d14 . . . d1N] Output ofSTBC block: [a11 a12 a13 a14 a15 a16 . . . a1N] [−b11* −c12* −d13* −b14*−c15* −d16* . . .] [b11 b12 b13 b14 b15 b16 . . . b1N] [a11* −d12* −c13*a14* −d15* −c16* . . .] [c11 c12 c13 c14 c15 c16 . . . c1N] [−d11* a12*b13* −d14* a15* b16* . . .] [d11 d12 d13 d14 d15 d16 . . . d1N] [c11*b12* a13* c14* b15* a16* . . .]

The same principles are applicable for any 2p×p STBC system.

It will be appreciated by those of ordinary skill in the art that theinvention can be embodied in other specific forms without departing fromthe spirit or essential character thereof. The illustrated embodimentsare therefore intended in all respects to be illustrative and notrestrictive. The scope of the invention is indicated by the appendedclaims rather than the foregoing description, and all changes the comewithin the spirit and range of equivalents thereof are intended to beembraced therein.

1. A method of transmitting data according to one of at least a firsttransmission mode using a single antenna and a second transmission modeusing multiple antennas using either a single antenna or multipleantennas depending on the transmission mode, comprising: whentransmitting data according the first transmission mode, performingblock interleaving of said data using a first interleaving method priorto transmission; and when transmitting data according to the secondtransmission mode: forming from a first data stream multiple second datastreams, successive bits in said first data stream being assigned torespective ones of said second data streams; performing blockinterleaving of multiple ones of said second data streams using a sameinterleaving method as said first interleaving method, during whichblock interleaving bits are grouped into symbols; and performing areordering of symbols such that symbols from one of said second datastreams are reordered as compared to symbols of another of said seconddata streams.
 2. The method of claim 1, comprising, for at least one ofsaid second data streams, for each data symbol, performing cyclicrotation of bits within the data symbol.
 3. The method of claim 2,comprising, for multiple ones of said second data streams, for each datasymbol, performing cyclic rotation of bits within the data symbol. 4.The method of claim 3, comprising performing cyclic rotation differentlyfor different ones of said second data streams.
 5. A data transmitterfor transmitting data according to one of at least a first transmissionmode using a single antenna and a second transmission mode usingmultiple antennas using either a single antenna or multiple antennas(ant_(—)1, ant_n) depending on the transmission mode, comprising: meansfor, when transmitting data according to the first transmission mode,performing block interleaving of said data using a first interleavingmethod prior to transmission; and when transmitting data according tothe second transmission mode: means for forming from a first data streammultiple second data streams (503), successive bits in said first datastream being assigned to respective ones of said second data streams;means for performing block interleaving of multiple ones of said seconddata streams using a same interleaving method as said first interleavingmethod during which block interleaving bits are grouped into symbols(511 a); and means for performing a reordering of symbols such thatsymbols from one of said second data streams are reordered as comparedto symbols of another of said second data streams (513 a).
 6. Theapparatus of claim 5, comprising means for, for at least one of saidsecond data streams, for each data symbol, performing cyclic rotation ofbits within the data symbol.
 7. The apparatus of claim 5, comprisingmeans for, for multiple ones of said second data streams, for each datasymbol, performing cyclic rotation of bits within the data symbol. 8.The apparatus of claim 7, wherein said means for performing cyclicrotation performs cyclic rotation of bits in a same way for differentones of said second data streams, further comprising means forperforming cyclic rotation differently for at least one of saiddifferent ones of said second data streams.
 9. The apparatus of claim 8,wherein said means for altering follows said means for performing cyclicrotation.
 10. The apparatus of claim 8, wherein said means for alteringprecedes said means for performing cyclic rotation.
 11. The apparatus ofclaim 8, wherein said means for altering precedes said means forperforming block interleaving.
 12. The apparatus of claim 11, whereinsaid means for altering is a block interleaver.